(1) Field of the Invention
The present invention generally relates to a time slot rearranging apparatus in a multiplexer, and more particularly to a time slot rearranging apparatus in which time slots of multiple signals in an optical subscriber line are rearranged so that a primary rate signal has a predetermined format.
(2) Description of Related Art
Recently, a system has been proposed in which digital multiple subscriber signals, each having an original format, are transmitted to a remote terminal, such as an exchange, via optical fibers. In this system, as a multiple signal output from the remote terminal at the primary rate has to have a predetermined format, time slots of the multiple signals in an optical subscriber line (the optical fibers) must be rearranged so that the primary rate multiple signal has a predetermined format.
First, a conventional system in which analog signals in a subscriber line (a metal line) are multiplexed will be described below.
FIG. 1A shows a conventional system. In FIG. 1A, the system is provided with twelve channel boards 61 (#1-#12). Two subscriber lines are accommodated in each of the channel boards 61 via an input port 60, and each of the channel boards 61 has a coder 62 for converting analog signals to digital signals and a three state gate 63 capable of being in a high-impedance state. Each of the channel boards 61 is coupled to a multiplexer board 66 by a bus 64 and a channel identifying line 65. Subscriber signals supplied to each of the channel boards 61 are coded into digital data and supplied to the multiplexer board 66 via the bus 64. The channel boards 61 are controlled by channel identifying signals supplied from the multiplexer board 66 via the channel identifying lines 65.
A primary rate multiple signal output from the multiplexer board 66 has a format as shown in FIG. 1B (a). In FIG. 1B (a), one frame of the multiple signal is shared by 24 channels. A bit "F" positioned at a lead end of the frame is used for synchronizing. In a case where the multiple signal is formed of a plurality of frames (e.g. twelve frames or twenty four frames) which is referred to as a multi-frame signal, bits "F" of respective frames form a synchronizing pattern (e.g. 0101 . . . ). After the bit "F", signals of 24 channels, each having 8 bits, are successively arranged.
The multiplexer board 66 outputs the channel identifying signals as shown in FIG. 1B (b)-(e). The channel identifying signals are successively activated. Each time one of the channel identifying signals is activated, a corresponding channel board 61 outputs a multiple signal including two channels. The multiplexer 66 rearranges time slots of the multiple signals successively supplied from the channel boards 61 so that a primary rate multiple signal including twenty four channels is output from the multiplexer board 66.
The channel board 61 (#1) is, for example, shared by a first channel (CH1) and a thirteenth channel (CH13), the channel board 61 (#2) is shared by a second channel (CH2) and a fourteenth channel (CH14), and other channel boards 61 (#3-#12) are respectively shared by channels different from each other. A channel identifying signal supplied from the multiplexer board 66 to the channel board 61 (#1) is activated at time slots corresponding to the first and thirteenth channels. At a time slot corresponding to the first channel (CH1), one of the subscriber signals is input to the channel board 61 (#1) via the input port 60 and converted into a digital signal by the coder 62. The digital signal is then supplied to the bus 64, to which a pull-up resistor is connected, via the gate 63. After this, during time slots corresponding to the channels CH2 through CH12, the gate 63 of the channel board 61 (#1) is in the high-impedance state. At a time slot corresponding to the thirteenth channel CH13, another subscriber signal is input to the channel board 61 (#1) and converted into a digital signal. In the other channel boards 61, at time slots corresponding to channels sharing each of the channel boards 61, subscriber signals are converted into digital signals by the coder 62, and the digital signals are supplied to the bus 64. As a result, a multiple signal having the format as shown in FIG. 1B (a) is supplied to the multiplexer board 66 via the bus 64. The multiplexer board 66 adds the bit "F" to the multiple signal, generates a multi-frame multiple signal, and carries out other controls.
A primary rate multiple signal may have two types of formats. In the first format, channels are arranged in an order of CH1, CH2, CH3, . . . , CH24. In the second format, channels are arranged in an order of CH1, CH13, CH2, CH14, CH3, CH15, . . . , CH12, CH24. The first format and second format are respectively often referred to as a sequential format and a D1D format.
The conventional subscriber signals described above are analog signals. Next, a proposed system in which the subscriber line is formed of optical fibers will be describe below with reference to FIGS. 2A, 2B and 2C.
FIG. 2A shows a system in which the subscriber line is formed of optical fibers. In FIG. 2A, subscriber terminals 70 such as telephone sets and data processing terminals are provided in this system. Digital signals are output at a rate, for example, of 64 kbps from the subscriber terminals 70 and multiplexed. The multiple signal is transmitted to a remote station 72 via an optical subscriber line 71 at a rate, for example, of 1.5 Mbps. A multiple digital signal in the optical subscriber line 71 may have an original format. It is preferable that the format of the multiple digital signal in the optical subscriber line 71 be similar to the format of the primary rate signal. FIG. 2B shows an example of a format of a digital signal in the optical subscriber line. A bit "F" used for synchronizing is positioned at a lead end of the signal, and after the bit "F", channels CH1, CH2, . . . and CH12 are arranged. In this example, only a few channels are simultaneously used and all of the twenty four channels are not used. Thus, time slots corresponding to channels CH13-CH24 after the channel CH12 are used for other information (of controlling and checking). The subscriber signal is a multi-frame signal including twelve frames.
Time slots of the signals, as shown in FIG. 2B, in the optical subscriber lines are rearranged in the remote station 72 so that a primary rate multiple signal has a predetermined format. An example of the sequential format of the primary rate multiple signal is shown in FIG. 2C. In FIG. 2C, a multi-frame signal includes twelve or twenty four frames each frame being formed as shown in FIG. 1B (a). The D1D format is not shown in FIG. 2C.
In a case where time slots of digital signals in the optical subscriber lines connected to the remote station are rearranged so that a primary rate multiple signal has a predetermined format, the conventional system as shown in FIG. 1A is not used as it is, because the conventional system as shown in FIG. 1A multiplexes analog signals.
In addition, the primary rate multiple signal may have either the sequential format or the D1D format. The format of the primary rate multiple signal depends on the equipments installed in each remote station. Thus, a system for rearranging the time slots of the subscriber signals must be changed in accordance with the type of the format (the sequential format or the D1D format) of the primary rate signal.